Method of determining a spectral route for a given connection in an optical telecommunications network

ABSTRACT

In order to determine a spectral route for a given connection in an optical telecommunications network (T) between a starting node (ON 1 ) and a destination node (ON 6 ) of the network, the method consists in: using a conventional routing method to determine one or more candidate spatial routes (Route i, Route  2 ) connecting the starting node (ON 1 ) to the destination node (ON 6 ), each candidate spatial route comprising a sequence of route segments, each segment connecting two nodes of the network directly and being adapted to support a plurality of spectral routes.

FIELD OF THE INVENTION

The invention relates to a method of determining a spectral route for agiven connection in an optical telecommunications network, and it alsorelates to a node for implementing the method. It relates moreparticularly to wavelength division multiplex (WDM) optical networksthat use a plurality of wavelengths to transmit a plurality of datastreams simultaneously on the same optical fibre.

BACKGROUND OF THE INVENTION

To set up a connection in this kind of network, it is necessary todetermine not only a spatial route, consisting of a sequence of routesegments connecting the starting node to the destination node, but alsoa spectral route, since each segment is able to support a plurality ofwavelengths each constituting a spectral route segment. Selecting aspectral route entails selecting the wavelength to be used, or thewavelengths to be used successively, on different segments along thespatial route. It is sometimes necessary to carry out operations on thesignal and/or the information transported, necessitating the addition ofdedicated equipment to the network and the use thereof. However, suchprocessing operations are costly, and it is therefore desirable to avoidthem as much as possible. They may relate to regeneration and/orwavelength conversion, which may be carried out by purely optical meansor using optical-electrical and electrical-optical conversion means.

The expression “wavelength continuity” refers to the use of the samewavelength from the starting node to the destination node, even ifoperations on the signal and/or the information transported requireoptical-electronic-optical conversion or 1R, 2R, or 3R regeneration.

The term “transparency” is used, a distinction being drawn betweendifferent types of transparency, depending on whetheroptical-electronic-optical conversion, wavelength conversion, 1R, 2R or3R regeneration, or a combination of these operations are avoided. Theaim is to avoid the route for the signal passing through“non-transparency points” or, if this is not possible, to minimize thenumber of non-transparency points the signal passes through. Forexample, in the case of a form of transparency consisting in the absenceof optical-electronic-optical conversion, the aim is to minimize thenumber of times the optical signal passes through opto-electronic andelectronic-optical interfaces. If it is not possible to avoid conversioncompletely, then a route is looked for that minimizes the number ofconversions needed.

There may additionally be connection capacity constraints orquality-of-service constraints that influence the selection of thespatial route and the spectral route.

At present, there is no satisfactory method of determining a transparentroute of the above kind. One method was proposed to the IETF inGeneralized MPLS—Signaling Functional Description, chapters 3.4 and 3.5,Expiration date: November 2001, Network Working Group, Internet Draft.

That prior art method consists in:

-   -   using a conventional routing method to determine a spatial route        connecting a starting node to a destination node and comprising        a sequence of route segments, each segment interconnecting two        nodes of the network directly;    -   determining a first set of wavelengths, in the starting node,        for communicating with the next node constituting the route,        i.e. the second node on the route;    -   determining, from the set of wavelengths proposed by the        preceding node, a second set of wavelengths, in the second node,        for communicating with the next node constituting the route,        i.e. the third node on the route;    -   in the n^(th) node, determining, from the set of wavelengths        proposed by the preceding node, an (n+1)^(th) set of        wavelengths, for communicating with the next node constituting        the route, i.e. the (n+1)^(th) node on the route; and so on, as        far as the destination node.

The object of the above prior art method is only to assure wavelengthcontinuity. The nodes do not propagate the sets of wavelengths becausethey do not need to do this to find a spectral route assuring wavelengthcontinuity (should one exist end to end).

Each node of the route may retain or reduce the set of wavelengths thatit inherits from the upstream node, according to the resources availablefor the connection to the downstream node. A transparent route isfinally set up if the resulting set contains at least one wavelength. Adrawback of that method is a high probability of blocking, since theselection made locally at each node may reduce options in downstreamnodes. That method therefore constitutes a sub-optimum solution, or evenno solution at all in some cases, when there is in fact an acceptablesolution, although it is not transparent end to end. Also, the aboveprior art method takes account of only one parameter: the continuity ofa given wavelength.

SUMMARY OF THE INVENTION

The object of the invention is to propose a method that does not havethe above drawbacks.

The invention consists in a method of determining a spectral route in anoptical telecommunications network between a starting node and adestination node of the network, the method being characterized in thatit consists in:

-   -   using a conventional routing method to determine at least one        candidate spatial route connecting the starting node to the        destination node, each candidate spatial route consisting of a        sequence of route segments, each segment connecting two nodes of        the network directly and being adapted to support a plurality of        wavelengths each constituting a spectral route segment;    -   collecting values of parameters characterizing all the spectral        route segments along each candidate spatial route; and    -   finally, using ah optimization method to process all the        collected parameter values to select a spectral route and the        spatial route that supports it by selecting the wavelength to be        used, or the wavelengths to be used successively, to connect the        starting node to the destination node.

The above method has the advantage of reducing the probability ofblocking resulting from the impossibility of finding a route, since itprovides, at any given time, knowledge of the parameter values for allthe segments constituting one or more candidate routes (especiallytransparency parameter values). This complete knowledge is used toselect more efficiently a transparent route or a route comprising aminimum number of non-transparency points. This overview leads to realoptimization, i.e. avoiding possible solutions being abandoned duringthe process.

In a preferred embodiment, to collect parameter values characterizingall route segments along each candidate spatial route, a route set-uprequest message is sent from the starting node to the destination nodeand parameter values are collected in the message as it passes througheach node along the candidate spatial route.

It should be observed that, in order to maximize the chances of findinga transparent route, this method may be applied several timessimultaneously on separate spatial routes to satisfy the same connectionset up request.

A preferred embodiment of the method of the invention is implemented inthe node at the end of the route whose setting up has been requested,which is called the destination node.

The method of the invention uses the signaling means of the network totransmit transparency parameter values, providing up-to-date values foreach new route set up request.

These parameter values do not relate only to wavelengths, and may relateto all other physical parameters of the connections between the nodes ofthe network.

In one embodiment, the parameters characterizing all the spectral routesegments along each candidate spatial route take account of transparencyconstraints.

In one embodiment, the parameters characterizing all the spectral routesegments along each candidate spatial route take account of connectioncapacity constraints.

In one embodiment, the parameters characterizing all the spectral routesegments along each candidate spatial route take account of quality ofservice constraints.

The invention also provides an optical network node for implementing amethod according to the invention, the node being characterized in thatit comprises management means for:

-   -   receiving a route set-up request message on a predetermined        spatial route passing through the node;    -   adding to the content of the message parameter values concerning        spectral routes supported by the spatial route segment        immediately upstream and/or downstream of the node on the        spatial route, together with parameter values concerning the        interfaces of the node; and    -   forwarding the message modified in this way to another node        situated on the spatial route segment immediately downstream of        the node and designated by routing information contained in the        message.

BRIEF SUMMARY OF THE DRAWINGS

The invention will be better understood and other features will becomeapparent in the light of the following description and the accompanyingdrawings, in which:

FIG. 1 shows one example of an optical network in which the method ofthe invention may be used;

FIG. 2 shows the execution of a first portion of the method of theinvention in the network example represented in FIG. 1; and

FIG. 3 shows the execution of a second portion of the method of theinvention in the network example represented in FIG. 1.

The network T shown by way of example in FIG. 1 comprises optical nodesON1 to ON6 interconnected by bidirectional or unidirectionalconnections:

ON1-ON2

ON1-ON3

ON1-ON4

ON2-ON6

ON3-ON5

ON3-ON6

In this example, the network T interconnects three client networks CNA,CNB, CNC which are connected to nodes ON1, ON4, ON6, respectively, atthe edge of the network T. The method of the invention is executed inthe network T and is totally independent of the number and nature of theclient networks.

DETAILED DESCRIPTION OF THE INVENTION

In one implementation of the method of the invention, a connectionset-up request CSR is sent by the client network CNA to the managementmeans of the node ON1 in order to set up a connection between the clientnetworks CNA and CNC. The request contains the identity of therequesting client network CNA and the identity of the requested clientnetwork CNC and indicates constraints on transparency, capacity, qualityof service, etc. The network T must determine a transparent route or, ifthis is not possible, a route comprising as few points ofnon-transparency as possible but still conforming to the capacity andquality of service constraints set for the connection. The opticaltransparency parameter constraints may be values of wavelength, spectralspacing, tolerance of non-linear effects (four-wave mixing, etc.), theobligatory absence of regeneration, etc.

FIG. 2 shows a first portion of this embodiment of the method of theinvention. The management means of the node ON1 translate the connectionset-up request CSR into a route set-up request RSR, i.e. translateconstraints referred to in the connection request into constraintsrelating to routing.

Using a conventional routing method, the management means determine oneor more spatial routes, referred to as candidate routes, connecting theclient network CNA to the client network CNC, as a function of thetopology and the connectivity of the network T. For example, they findtwo candidate spatial routes, Route 1 and Route 2, which are validbecause they satisfy all the routing constraints referred to in theoriginal connection set-up request CSR.

Route 1=ON1, ON4, ON5, ON6

Route 2=ON1, ON3, ON6

The management means of the node ON1 then forward the route set-uprequest RSR to the node ON6, and two copies RSR1 and RSR2 of the requestare routed simultaneously on the two routes Route 1 and Route 2. Therouting along these two routes is controlled by the management means ofthe starting node ON1 on furnishing the request to the signaling meansof the network T. The copy RSR1 passes first through the node ON4 on theroute Route 1 and the copy RSR2 passes through the node ON3 on the routeRoute 2.

Each node ON1, ON4, ONS through which the route set-up request RSR1passes adds to the content of the request parameter values relating tothe route segment immediately upstream and/or downstream of the node onthe spatial route concerned, together with parameter values concerningthe interfaces of the node, these values corresponding to parametersreferred to in the request, in particular optical transparencyparameters. Each node ON1, ON3 through which the route set-up requestRSR2 passes performs the same action on the request RSR2. the linkbetween ON3 and ON6. These parameter values may equally relate to thedownstream connection and the upstream connection of the node. Finally,the two copies RSR1 and RSR2 of the route set-up request reach thedestination node ON6.

In a first embodiment, the data collected in these two copies isprocessed by the management means of the destination node ON6 todetermine an optimum combination of spectral route segments between thenodes ON1 and ON6 along each of the spatial routes taken by theconnection requests. This data is processed using an optimizationalgorithm that minimizes a cost function taking into account all theparameter values collected. This algorithm may employ a shortest pathalgorithm such as the Dijkstra algorithm.

If at least one transparent route is possible, the optimizationalgorithm finds a transparent route. If no transparent route ispossible, the optimization algorithm determines a route comprising thefewest possible points of non-transparency, i.e. an optimum combinationof transparent sub-paths.

FIG. 3 represents a second portion of the method of the invention. Afterdetermining the optimum spectral route in the manner described above,the destination node ON6 sends a route establishment message PEM to thestarting node ON1 and a route set-up request received acknowledgementmessage ACK to the next node on the spatial route that supports theselected spectral route (Route 2), which is the node ON3 in thisexample. This message contains a list of the nodes constituting theroute that has been determined, and in this example Route 2 is made upof the nodes ON1-ON3-ON6.

The destination node ON6 sends a route release message RR to all theother immediately adjacent nodes along spatial routes that have not beenadopted, in this instance Route 1. In this example, the route releasemessage RR is sent to the node ON5, which forwards it in the directionof the node that was the source of the route set-up request (node ON1).

If the algorithm concludes that no route is possible (even anon-transparent route), the destination node ON6 sends a route releasemessage to all of the nodes on the routes Route 1 and Route 2, i.e. thenodes ON1, ON3, ON4, ON5.

In a second embodiment, the two copies RSR1 and RSR2 of the route set-uprequest (containing all the data collected) are not processed in thedestination node ON6, but are returned from the destination node ON6 tothe starting node ON1, or to a central unit somewhere in the network T,to be processed there. If the algorithm concludes that no route ispossible (even a non-transparent route), the node ON1, respectively thecentral unit, sends a route release message to all of the nodes situatedalong the routes Route 1 and Route 2, i.e. the nodes ON1, ON3, ON4, ON5.

The first embodiment has the advantage that it avoids forwarding all ofthe data collected to the starting node or a central processing unit.This avoids occupying network resources for such forwarding.

1. A method of determining a spectral route in an opticaltelecommunications network between a starting node and a destinationnode of the network, the method comprising: determining at least onecandidate spatial route to connect the starting node to the destinationnode via network nodes disposed intermediate between the starting nodeand the destination node, the candidate spatial route consisting of asequence of spatial route segments, each spatial route segmentconnecting two nodes of the network directly and being adapted tosupport a plurality of wavelengths, each wavelength constituting aspectral route segment; sending a route set-up request message from thestarting node to the destination node via the candidate spatial route;collecting values of parameters characterizing the spectral routesegments, which values of the parameters include values of opticaltransparency parameters, in the message as the message traverses thecandidate spatial route; receiving the message with the collectedparameters values in the destination node; and using an optimizationmethod to process the collected parameters values in the destinationnode upon receipt of the message to select the spectral route and thespatial route that supports the selected spectral route by selecting thewavelength to be used, or the wavelengths to be used successively, tospectrally connect the starting node to the destination node.
 2. Themethod according to claim 1, wherein the parameters characterizing allof the spectral route segments along each candidate spatial route takeaccount of transparency constraints.
 3. The method according to claim 1,wherein the parameters characterizing all of the spectral route segmentsalong each candidate spatial route take account of connection capacityconstraints.
 4. The method according to claim 1, wherein the parameterscharacterizing all of the spectral route segments along each candidatespatial route take account of quality of service constraints.
 5. Anoptical network node for implementing the method according to claim 1,comprising management means for: receiving a route set-up requestmessage on a predetermined spatial route passing through the node;adding to the content of the message parameter values concerningspectral routes supported by the spatial route segment immediately oneof upstream and downstream of the node on the spatial route, togetherwith parameter values concerning interfaces of the node; and forwardingthe message modified in this way to another node situated on the spatialroute segment immediately downstream of the node and designated byrouting information contained in the message.
 6. The optical networknode according to claim 5, wherein the method further comprises:determining sets of wavelengths available for a connection from theoptical network node to a downstream node along the spatial routesegments, wherein the values of the collected parameters includeidentifications of the determined sets of available wavelengths.
 7. Anoptical network node for implementing the method according to claim 1,the node comprising management means for: receiving at least one messagecontaining parameters values collected along a candidate spatial routeconnecting the starting node to the node; and using an optimizationmethod to process the collected parameters values to select a spectralroute by selecting the wavelength to be used, or the wavelengths to beused successively, and connect the starting node to the optical networknode.
 8. The optical network node according to claim 7, wherein themethod further comprises: determining sets of wavelengths availablealong the spatial route segments, from the starting node to thedestination node, wherein the values of the collected parameters includeidentifications of the determined sets of available wavelengths.
 9. Theoptical network node according to claim 8, wherein the method furthercomprises: selecting the spectral route as a transparent route whichuses the same wavelength from the starting node to the destination nodeand lacks optical to electrical to optical conversion.
 10. The opticalnetwork node according to claim 8, wherein the method further comprises:selecting the spectral route as a combination of transparent sub-pathswhich spectrally connect one node to another node, wherein eachtransparent sub-path uses the same wavelength from the one node to theanother node and lacks optical to electrical to optical conversion. 11.The method according to claim 1, further comprising: determining sets ofwavelengths available along the spatial route segments, from thestarting node to the destination node, wherein the values of thecollected parameters include identifications of the determined sets ofavailable wavelengths.
 12. The method according to claim 11, furthercomprising: selecting the spectral route as a transparent route, whichuses the same wavelength from the starting node to the destination nodeand lacks optical to electrical to optical conversion.
 13. The methodaccording to claim 11, further comprising: selecting the spectral routeas a combination of transparent sub-paths which spectrally connect onenode to another node, wherein each transparent sub-path uses the samewavelength from the one node to the another node and lacks optical toelectrical to optical conversion.
 14. The method according to claim 1,wherein using the optimization method comprises: processing the valuesof the optical transparency parameters, collected in the receivedmessage; minimizing a cost function based on the processed values of theoptical transparency parameters; and determining a shortest spectralroute including an optically transparent path from the source node tothe destination node.
 15. The method according to claim 14, whereinminimizing the cost function comprises: using a Dijkstra's algorithm.16. The method according to claim 14, wherein the optically transparentpath does not lie through opto-electronic and electronic-opticalinterfaces.
 17. The method according to claim 1, wherein using theoptimization method comprises: processing the values of the opticaltransparency parameters, collected in the received message; determininga presence of an optically transparent path from the source node to thedestination node; and one of: informing the source node of thedetermined optically transparent path, and determining an optimal pathbetween the source node and the destination node, which optimal pathincludes the least possible points of non-transparency and informing thesource node of the determined optimal path.